Wait to increment counters until PHP shutdown

If all rrdtool counters are enabled, then the rrdtool module can increment a counter in memcache for every single Drupal hook. This will have some definite impact on performance and page load times on a busy website.

Let's make the increment function build a static array of all the counters that need to be incremented, but wait to actually increment them on PHP shutdown after the page has already been served up. This should be simple enough, just need to add an additional parameter to the increment function which we only set when we call it from the php shutdown.

If php has a a multi-increment function, all the better.